1. Field of the Invention
The present invention relates to an apparatus and a method for inspecting an object to be inspected (i.e., an inspection target). More particularly, the present invention is adapted for an optical defect inspection apparatus and method for inspecting foreign matters, defects, etc. on inspected objects in manufacturing processes of, e.g., semiconductor devices, flat panel displays, magnetic disks, and masks.
2. Description of the Related Art
The capability of detecting finer defects is demanded in an optical defect inspection apparatus for illuminating a light to an object to be inspected, such as a semiconductor device and a flat panel display, and measuring the light reflected or scattered from the object. To meet such a demand, using a light source with higher luminance is required and a laser beam is mainly used as the light source. One known inspection apparatus using the laser beam is disclosed in Patent Document 1 (JP, A 2005-156537). On the other hand, with an increase of needs for unified management of inspection conditions, stability and reproducibility of the inspection apparatus are also demanded in addition to higher detection sensitivity.
When a laser beam is employed as the light source, a thin beam with a high illumination density has to be used until reaching an ND (Neutral Density) filter, which is used to adjust the light quantity (intensity), due to the necessity of avoiding an increase in size of an ND filer mechanism. Also, from the viewpoint of ensuring flexibility in apparatus layout, the inspection apparatus is usually constructed such that a thin beam with a high illumination density is reflected several times within an apparatus housing by reflecting mirrors (plane mirrors). In such a case, when the laser beam is illuminated to the same position of the plane mirror for a long time, there occurs a problem in stability that the mirror surface in an illuminated area is deteriorated, the reflection is reduced, and the illumination intensity cannot be held at a required level.
If the reflectance is reduced, the plane mirror has to be replaced. After the replacement, a slight deviation of an optical axis is unavoidable. To compensate for such a deviation of the optical axis, Patent Document 2 (JP, A 2004-45111) discloses the provision of a mechanism for changing an angle of a plane mirror.
Further, in addition to the above-mentioned change of the illumination intensity due to deterioration of the plane mirror, dust generated from an actuator and a variation of the focal length caused by flexing of the inspected object become factors acting to reduce stability and reproducibility of the inspection apparatus. One example of actuators having a structure to cope with generation of dust is disclosed in Patent Document 3 (JP, A 7-235579).